This invention relates to a method and apparatus for reducing the sodium chloride content of potassium chloride by a digestive crystallizing process. In particular, it relates to such a process where smaller and more impure feed crystals are slurried in an aqueous medium with larger and more pure product crystals so that the feed crystals dissolve and purer potassium chloride grows on the larger product crystals.
Potassium chloride is sold according to the amount of sodium chloride it contains, the most expensive grades typically containing the least sodium chloride. Mine run potassium chloride contains about 60% sodium chloride; agricultural grade contains about 3%; industrial grade contains 1000 to 3000 ppm; and chemical grade contains about 200 ppm. The presence of sodium chloride makes agricultural grade potassium chloride unsuitable for use in many industrial processes. For example, potassium chloride is used in electrolytic cells to produce potassium hydroxide. The presence of excessive concentrations of sodium chloride in the potassium chloride causes contamination of the potassium hydroxide product with sodium hydroxide. This is very undesirable in several important uses for potassium hydroxide.
Industrial grade potassium chloride can presently be made from lower grades of potassium chloride by recrystallization. (See, for example, U.S. Pat. Nos. 3,644,102 and 3,440,023.) In recrystallization, the low grade potassium chloride is dissolved and water is evaporated to preferentially precipitate potassium chloride while the sodium chloride remains in solution. Although recrystallization is an effective process, a great deal of energy is required to evaporate the water, and therefore it is very costly.